As the performance and density of electronic devices have risen in recent years, while the size of these devices has decreased, an issue has been how to efficiently radiate the heat generated from these devices and the electronic components of which they are made up. The need to deal with this heat is particularly acute with semiconductor lasers and CPUs, which are the heart of computers.
For cooling to be carried out efficiently, there needs to be a good combination of convection, radiation, conduction, and so forth. When cooling the above-mentioned electronic components, it is effective to cool them by transferring the heat to a lower-temperature region primarily by thermal conduction.
Heat radiating materials composed of metals with high thermal conductivity, such as copper (Cu) or aluminum (Al), are often used as the heat radiating means for electronic devices, electronic components, and so forth.
However, as elements are being made increasingly smaller, or generate more heat, it is becoming impossible to deal with this problem with conventional heat radiating materials. Consequently, there is a need for the development of a high thermal conductivity member with better thermal conductivity. The development of high thermal conductivity members with greater freedom of geometrical shape is also to be desired.
In light of this situation, graphite, which is composed of carbon (C), holds much promise as a replacement for the above-mentioned heat radiating materials, because it has outstanding heat resistance, chemical resistance, electrical conductivity, and so on, and also has high thermal conductivity.
FIG. 4 shows the crystal structure of graphite. More specifically, graphite has a structure made up of layered six-membered ring planar structures (graphene layers) 7 composed of six carbon (C) atoms. Because of this crystal structure, graphite is characterized by two directions, namely, a direction perpendicular to the graphene layers (c axial direction), and a direction parallel to the graphene layers (a-b axial directions, that is, directions perpendicular to the c axis).
For instance, in an ideal graphite crystal, the thermal conductivity κ⊥ in a direction perpendicular to the graphene layers 7 (the c axial direction, hereinafter also referred to as the “layer direction” (thickness direction)) is not very high (10 W/m·k or less). In contrast, the thermal conductivity κ∥ in a direction parallel to the graphene layers 7 (the a-b axial directions, hereinafter also referred to as the “planar direction”) is over 1000 W/m·k. Such thermal conductivity is more than twice that of copper (κ(Cu) ˜350-400 W/m·k), and more than four times that of aluminum (κ(Al) ˜200-250 W/m·k). Actually, it has been reported that κ∥=about 200 to 900 W/m·k with monocrystalline graphite, and κ∥=about 50 to 400 W/m·k with other graphite materials. Accordingly, various thermally conductive graphite materials that take advantage of high thermal conductivity in this planar direction have already been proposed.
For example, there have been reports of thermally conductive members whose thermal conduction characteristics have been improved by dispersing graphite powder in a polymer matrix or a silicone resin (Japanese Published Patent Applications S61-145266, H01-040586, H03-009552, H09-102562, H09-283955, 2002-299534, 2002-363421, 2003-105108, etc.). FIG. 10 is a simplified diagram of these thermally conductive members that have been disclosed (Conventional Example 1). In Conventional Example 1, it has been reported that thermal conductivity can be improved (κ˜several tens W/m·k) by dispersing and orienting a graphite powder 12 with high thermal conductivity in the planar direction in a polymer or other matrix 11.
A member produced by the compression molding of graphite flakes along with a polymer binder has also been reported (Japanese Published Patent Applications H01-009869 and H11-001621). Further, there has been a report of a member produced by hot pressing the product of compounding a metal powder and a crystalline carbon material (Japanese Published Patent Application H10-168502). FIG. 11 is a simplified diagram of these thermally conductive members that have been disclosed (Conventional Example 2). In Conventional Example 2, a thermally conductive member 13 (κ=400 to 970 W/m·k) composed of graphite with high thermal conductivity is produced by compression molding a graphite powder 14 (or mixture of metal and graphite powder) with high thermal conductivity in the planar direction.
Sheet-form thermally conductive members composed of just graphite with high in-plane orientation are also known (Japanese Published Patent Applications S58-147087, S60-012747, and H07-109171). FIG. 12 is a simplified diagram of the thermally conductive members disclosed in these publications (Conventional Example 3). With Conventional Example 3, an organic polymer sheet is baked, which provides a thermally conductive member (κ=600 to 1000 W/m·k) composed of a graphite structure with extremely high in-plane orientation.
Thus, thermally conductive members that make use of graphite as a thermally conductive substance are characterized by superior thermal conductivity, freedom of shape, and so forth as compared to conventional members made up of copper (Cu) or aluminum (Al).